Create a todo when a comment starts with TODO
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### Problem to solve
Make todos even easier to create.
### Intended users
gitlab-ce~9335216
### Proposal
If a comment starts with `TODO`, create a real todo in the user's todo list that links to the comment. For example:

### Further details
A user can already can create a todo from an issue or merge request using the "Add todo" button at in the issue's sidebar:

However, this todo lacks context - it doesn't link to a specific comment or provide any details about _why_ the issue or merge request was marked as a todo.
### Documentation
If this feature was implemented, this page would need a small update explaining this feature: https://docs.gitlab.com/ee/workflow/todos.html
### What does success look like, and how can we measure that?
If people use it!
issue